This invention relates to communication circuits that include an optical transmission link and that are operable to transmit analog data; more particularly, the invention relates to such a circuit that includes signal transmission means which are capable of transmitting an analog data signal free of excessive ripple distortion. Also, an analog data communication circuit constructed according to the invention is capable of affording an indication of a failure in its optical transmission link.
A well known method for optically transmitting analog data is to encode an analog signal to digital form in order to avoid the amplitude distortions that can influence optical signals due to the many variable factors frequently encountered in optical data transmission. Typically, the analog data signal is converted to a string of voltage pulses the frequency of which is determined as a function of the input signal voltage amplitude. In the last several years voltage-to-frequency (V/F) converters that are stable in operation and have good resloution have been readily available commercially at relatively low cost. Consequently, the use of such converters in analog data transmission circuits has become a favored means for converting an analog signal to a desired string of voltage pulses. In such circuits, the variable frequency voltage pulse string is then converted to an optical signal which is transmitted through a fiber optic light guide to a suitable photo-detector, which converts the signal back to another string of voltage pulses. In order to reconstruct the signal a frequency-to-voltage (F/V) converter, which is typically a commercially available V/F converter used in inverse configuration, is typically used. The F/V converter produces an output voltage that varies in amplitude as a function of the frequency of the input signal.
A recognized disadvantage of this well-known method for optically transmitting analog data signals is that the output ripple of such a communication circuit is inversely proportional to the input frequency. Accordingly, it is normally found to be necessary to integrate the pulses over a long period of time in order to achieve an acceptable level of output ripple. The use of such long integrator time constants makes this a very slow response time technique. In order to improve the response time, a variety of complex circuits have been developed for use in place of simple integrator circuits, but it remains very desirable to afford a more efficient means for extending the useful range of operation of such well known optical data communication circuits by making them relatively free of excessive output ripple, particularly at the low frequency ranges where the output ripple distortion is normally the most severe.